1. Field of the Invention
This invention relates to methods of forming cartons, in which a carton blank of uncoated board is erected and selected parts thereof are adhered to cooperating parts of the blank to form a carton.
2. Description of the Prior Art
It is known to erect a carton by means of a punch which is forced through the plane of the blank and which cooperates with a die arrangement having pressure members to erect the carton and to press selected parts of the blank against cooperating parts of the blank to seal them together. Examples of this method, using thermoplastic adhesives or thermoplastic coatings on the blanks, are described in our British Patent Specifications Nos. 928135 and 1187258. The latter also describes how the selected and cooperating parts of the carton blank can be pre-heated by hot air jets to soften and render tacky a thermoplastic coating thereon, to assist the sealing.
For erecting cartons from blanks of uncoated board, it has been normal practice to apply by jet or brush to the appropriate parts of the carton blank a water-based adhesive, e.g. of aqueous-dispersed polyvinyl acetate. The blank, unheated, is thereafter erected to carton form in a punch and cooperating die arrangement, whereafter it is passed through a long pressure section by which pressure is maintained on the selected and cooperating parts of the erected carton for a time, typically one second in duration, sufficient for the adhesive to dry. As an alternative to the use of a pressure section following the punch and die arrangement, the erected carton may be held for the necessary drying period in the punch and die arrangement itself. No specially provided pressure section is then required, but the throughput of the punch and die arrangement is correspondingly limited.
Another type of adhesive which has been used in the erection of cartons from blanks of uncoated board is a hot melt adhesive which is applied in molten form to the appropriate parts of the carton blank before the blank enters the punch and die arrangement. Such an adhesive is substantially instantaneous in its action, so that no holding or dwell period is required to allow it to set. However, it is expensive in comparison with water-based adhesives and moreover is unsatisfactory for use with some types of board, e.g. grease resistant board, which are uncoated but include additives to the furnish from which they are made.
The known methods have either been wasteful of adhesive and subject to severe throughput limitations (in the case of water-based adhesive), or, in the case of hot melt adhesive, have employed a high cost adhesive and have been unsatisfactory for some types of board.
It has been proposed in European Patent Application No. 79200702.3 (Publication No. 0017678 A1) to form a heat-resistant carton by coating at least one of the carton surfaces with a heat-activated thermosetting adhesive, then spraying a water-based adhesive emulsion (e.g. an aqueous emulsion of polyvinyl acetate) on the coated surfaces, heating at least the water-based adhesive coated surface (e.g. by a stream of heated air) to at least partially dry the water-based adhesive, placing the mating carton surfaces in contact with each other, and applying pressure for a time sufficient to seal the contacting surfaces together. It is said that the combination of thermosetting adhesive and water-based adhesive enables the mating surfaces of the carton to be rapidly sealed by automatic carton sealing apparatus.